1 Field of the Invention
This invention relates to in situ recovery of shale oil, and more particularly to techniques for stabilizing the roof of underground workings adjacent an in situ oil shale retort.
2. Description of the Prior Art
The presence of large deposits of oil shale in the semiarid high plateau region of the Western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits have been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; 4,043,598; and 4,192,554, which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded for forming a stationary fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the fragmented mass and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous material to produce heat, combustion gas, and combusted oil shale. By continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
In developing a mining system for a tract of in situ oil shale retorts, a number of factors must be considered. These include maximizing the amount of resource recovery, avoiding substantial subsidence of overburden, stabilizing drift systems to avoid safety hazards to workers present in the drift systems, and reducing mining and construction costs to a reasonable level.
In developing a mining system, there is a trade-off between retorting as much oil shale as possible to maximize resource recovery, and leaving sufficient unrecovered oil shale in the supporting pillars of unfragmented formation for supporting the weight of the overburden to avoid substantial subsidence. Subsidence can result in fracturing of overburden with consequent leakage of water from overlying aquifers into retort or mining areas, leakage of gas from completed retorts, and leakage of air into retorts during retorting operations. Such subsidence can occur when the extraction ratio in the tract is large and the remaining unfragmented formation is not sufficient for supporting the weight of the overburden.
A mining system also should provide stability in underground workings, such as drifts, where operating personnel are present, so that safety hazards, such as dangerous rock falls, can be avoided.
In some mining systems, moderate subsidence of overburden is tolerated. For example, U.S. Pat. No. 4,176,882 to Studebaker et al discloses a "controlled subsidence" technique for forming a tract of in situ retorts, in which moderate, but controlled, subsidence of overburden is permitted. In this system, partitions of unfragmented formation between adjacent retorts do not support overburden loads, resulting in a controlled amount of subsidence of overburden. In this technique, the partitions that separate adjacent retorts are about 30 feet thick. Rather than being load-bearing, such partitions yield under the load of the overburden; that is, the partitions support substantially the same proportionate amount of load of the overburden as the adjacent fragmented masses. In such a controlled subsidence technique, resource recovery is high because the retorts are closely spaced, separated only by the thin, non-load-bearing partitions. By allowing moderate subsidence to occur, the intent is to prevent abrupt fracture or shearing of overburden. U.S. Pat. No. 4,140,343 to Mills shows a similar controlled subsidence arrangement for a tract of in situ retorts, in which non-load-bearing pillars about 50 feet thick separate adjacent groups of retorts, while thinner partitions about 25 feet thick separate retorts within each group from one another.
As an alternative to a controlled subsidence technique, "non-subsiding" mining systems also have been proposed for developing a tract of in situ oil shale retorts. In a non-subsiding technique, relatively thick load-bearing barrier pillars of unfragmented formation are left between adjacent groups of in situ retorts. Following formation of fragmented masses within such a group of in situ retorts, the load normally supported by unfragmented formation within the retort sites is transferred to the load-bearing barrier pillars adjacent the groups of retorts. The pillars support the overburden loads sufficiently to minimize subsidence of overburden. This system can have high resource recovery because the barrier pillars can later be retorted in a second pass of a "two-pass" retorting operation. Such a two-pass system is described in U.S. Pat. No. 4,219,237 to Sisemore.
A mining system for developing a tract of in situ oil shale retorts also must be economically feasible. For example, the mining and construction costs involved in preparing a system of in situ retorts can be reduced tremendously by eliminating excavation of drift systems at one or more levels within the retort system. For instance, some retort mining systems have a separate air level drift system excavated at an elevation above each row of retorts for supplying air to the retorts during subsequent retorting operations. It is common to also provide one or more drift systems at a production level at an elevation near the bottom of the retorts in each row for withdrawing liquid and gaseous products of retorting.
The present invention provides a "non-subsiding" system of in situ oil shale retorts. This system is economical in that a single air inlet drift system supplies air to a pair of adjacent rows of retorts during retorting operations, rather than providing a separate air inlet drift for supplying air to the retorts in each row during retorting operations. A single production level drift system also can be provided for later collecting products of retorting from retorts in a pair of adjacent rows of retorts during retorting operations. The air inlet drifts and production level drifts are offset laterally from the side boundaries of the retorts rather than being directly above or below the retorts. In such an arrangement, it has been discovered that the roof of the air inlet drifts and especially the production level drifts can be subject to instability problems. The present mining system employs techniques for stabilizing the drifts communicating with the retorts to assure that hazardous conditions, such as dangerous rock falls or cracking into the retorts, are not likely.